A material of a printed wiring substrate for transmitting a high-frequency signal is required to have excellent dielectric properties in order to improve signal transmission characteristics. In recent years, for applications such as high-capacity communication equipment, a smartphone antenna module, a material for a cable system of a notebook personal computer, a material for millimeter wave radar, and car automatic brake system related equipment, demands for better dielectric properties, heat resistance, low stress, water resistance, adhesive property, and the like are increased for an electronic circuit board.
As an electronic circuit board in the related art, for example, a substrate is widely used that is obtained by curing a resin impregnated material which is obtained by impregnating glass fibers represented by frame retardant type 4 (FR4) or the like as a base material with a low dielectric constant epoxy resin as a matrix (PTL 1). However, the substrate does not lower the dielectric constant so much, and further improvement of the transmission characteristics of the signal is required.
In addition, a substrate having a low dielectric constant fluorine-based resin as a matrix dielectric layer is proposed (PTL 2), but it is known that the substrate has problems in adhesive property between the fluorine-based resin and the glass cloth layer, and in adhesive property to the copper foil.
In addition, a substrate using a fluororesin such as polytetrafluoroethylene (PTFE) or the like as a base material instead of glass fibers, and an epoxy resin as a matrix is proposed (PTL 3). Since PTFE has an excellent balance between dielectric properties and heat resistance, PTFE is widely used for circuit boards, or the like in high-frequency fields including this field. Moreover, PTFE also can withstand processes such as a solder reflow furnace so that these substrates have excellent dielectric properties to some extent, and are used, for example, for a circuit board of a large computer, or the like. Specifically, in a case where expanded porous PTFE (expanded PTFE or ePTFE) serving as a porous film of PTFE is used, adhesive property to epoxy resin is not high, but when impregnated with an epoxy resin forming a hard cured product, an integrated state in which the epoxy resin sufficiently wraps around to the inside of the porous film is obtained. Therefore, a substrate material having excellent dielectric properties to some extent can be obtained due to combination effect of both.
However, if even a slight detachment occurs between the expanded porous PTFE and the epoxy resin due to impact of earthquakes, drops, and the like regardless of the magnitude during the manufacturing process and actual use, a risk that reliability is lowered over time due to the influence of humidity, or the like, may happen as a great weak point so that it is not widely used for a circuit board of a mobile phone.
A liquid crystal polymer (hereinafter, referred to as “LCP” in some cases) is used as another material excellent in dielectric properties in the related art. However, LCP having heat resistance enough to withstand the solder reflow process at approximately 260° C. requires treatment at a temperature around 300° C. or higher, which causes a problem of extremely poor processability. In addition, in the final stage of the manufacture of the printed circuit board, in a case where a defect is found in the obtained product even if the defect is small, although the product is heated for a short time to approximately 400° C. locally with manual soldering, there is also a problem that it is impossible to correct a wiring board using LCP with manual soldering due to a heat resistance problem. In addition, LCP has poor adhesion to copper foil, so when laminating the LCP and the copper foil, in order to obtain sufficient adhesion, it is necessary to previously form a relatively large uneven shape on the surface of the copper foil, and to heat and press the same to laminate to the smooth surface of the LCP. However, when the uneven shape increases, the transmission distance of the signal increases, which causes a problem that transmission loss occurs. That is, although the LCP has excellent dielectric properties than the epoxy resin, it has problems to be solved as a substrate material for high-frequency signal circuits.
In addition, a maleimide compound such as a bismaleimide compound (hereinafter, referred to as “BMI” in some cases) is known to have high heat resistance, and in recent years the appearance of compounds having excellent dielectric properties is known (PTLs 4 to 6).
However, even if the bismaleimide compound is a bismaleimide compound having relatively excellent dielectric properties as described above, the strength of the bismaleimide compound itself is insufficient for application to the above applications. On the other hand, when combined with glass fibers in order to increase the strength of the bismaleimide compound, although the strength is increased, the dielectric properties are lowered, therefore there is a problem that the dielectric properties of the bismaleimide compound are canceled out.
In addition, it is proposed to prepare a substrate material using a known bismaleimide compound, and tetrafluoroethylene is mentioned as one of the options of the base material, but in the expressions that prepreg can be produced by combining a bismaleimide compound and a woven fabric or nonwoven fabric, PTFE is only included as an example of woven fabric or nonwoven fabric material, which is different from the gist of the present invention (PTL 7).
In addition, the resin impregnated material used for the substrate and the composite serving as the cured product thereof can be applied to applications other than the substrate, for example, these can be used for a structure such as a radome used for protecting a radar antenna for aircraft control from wind and rain. Materials constituting the radome are required to have low dielectric properties so that the strength does not decrease when the radar wave passes through the radome. As such a material, a fluorine-based resin having a low dielectric constant is used, and the above polytetrafluoroethylene (PTFE) is used in this field.
However, the strength of the fluorine-based resin itself is not high, and further, in order to obtain a thin sheet using the PTFE resin, it is necessary to manufacture the thin sheet by thinly slicing a hard massive PTFE resin, and user-friendly sheets that are flexible, thin, cloth-like, and durable have not been obtained until now.